In my experience if you don't have good mains pressure you won't get hot water from a straight forward Mains fed electric shower - plenty of water but no hot water. Was the water turned on anywhere else in the house during the shower? washing machine, garden hose? could there be a leak anywhere? I had this problem for a while and it turned out there was a hose outside left on, once I turned that off I had hot water.
If your water pressure is ok, most likely a faulty temp switch which usually resides on top of the cyinder within the shower unit.
Power to element goes through this switch so faulty switch = no power to element = no hot water.
Part costs about €15 and is easy enough DIY to fit.